Method of producing austenitic iron/carbon/manganese steel sheets having a high strength and excellent toughness and being suitable for cold forming, and sheets thus produced

1. A process for manufacturing a hot-rolled sheet made of iron/carbon/manganese steel, in which:

providing a semifinished product cast from a steel whose chemical composition comprises, the contents being expressed by weight:

0.5%≦C≦0.7%

17%≦Mn≦24%

Si≦3%

Al≦0.050%

S≦0.030%

P≦0.080%

N≦0.1%,

and, optionally, one or more elements such that:

Cr≦1%

Mo≦0.40%

Ni≦1%

Cu≦5%

Ti≦0.50%

Nb≦0.50%

V≦0.50%,

the composition further comprising iron and inevitable impurities resulting from the smelting;

heating the semifinished product to a temperature between 1100 and 1300° C.;

hot rolling the semifinished product so an end-of-rolling temperature is 890° C. or higher and precipitation of AIN is prevented;

observing a time delay between said end of rolling and a subsequent rapid cooling operation, in such a way that the point defined by said time delay and said end-of-rolling temperature lies within an area defined by the ABCD′E′F′A plot of FIG. 1 ;

coiling said sheet at a temperature below 580° C. to obtain a coiled hot-rolled sheet;

cold deforming said coiled hot-rolled sheet with an equivalent deformation ratio of 30% or less, without further annealing.

2. The process as claimed in claim 1 , wherein said semifinished product is cast in the form of thin strip, by being cast between steel rolls.

3. The process as claimed in claim 1 , wherein said coiled hot-rolled sheet is subjected to a cold deformation operation with an equivalent deformation ratio of between 1 and 30%.

4. The process as claimed in claim 1 , wherein a delay is observed between said end of rolling and a subsequent rapid cooling operation, in such a way that the point defined by said delay and said end-of-rolling temperature lies within an area defined by the ABCDEFA plot of FIG. 1 .

5. A process for manufacturing a cold-rolled austenitic iron/carbon/manganese steel sheet, comprising:

providing a semifinished product cast from a steel whose chemical composition comprises, the contents being expressed by weight:

0.5%≦C≦0.7%

17%≦Mn≦24%

Si≦3%

Al≦0.050%

S≦0.030%

P≦0.080%

N≦0.1%,

the composition further comprising iron and inevitable impurities resulting from the smelting;

heating the semifinished product to a temperature between 1100 and 1300° C.;

hot rolling the semifinished product so an end-of-rolling temperature is 890° C. or higher and precipitation of AIN is prevented;

observing a time delay between said end of rolling and a subsequent rapid cooling operation, in such a way that the point defined by said time delay and said end-of-rolling temperature lies within an area defined by the ABCD′E′F′A plot of FIG. 1 ;

coiling said sheet at a temperature below 580° C. to obtain a coiled hot-rolled sheet, the coiled hot-rolled sheet having an austenitic grain size of less than 18 microns; and

subjecting the coiled hot-rolled sheet to at least one cold-rolling followed by an annealing operation, each cold-rolling comprising:

cold-rolling said sheet and

carrying out an annealing operation at a temperature of between 600 and 900° C. for a time of between 10 and 500 seconds, followed by a cooling operation, the cooling rate being greater than 0.5° C/s.

6. The process for manufacturing a cold-rolled sheet as claimed in claim 5 , wherein, after the final annealing, a cold-deformation operation is carried out with an equivalent deformation ratio of 30% or less.

7. The process for manufacturing a cold-rolled sheet as claimed in claim 5 wherein the composition includes one or more of the following elements such that:

Cr≦1%

Mo≦0.40%

Ni≦1%

Cu≦5%

Ti≦0.50%

Nb≦0.50%

V≦0.50%.

8. A process for manufacturing a steel sheet, comprising:

smelting a semi-finished product from a steel;

casting the semifinished product;

heating the semifinished product to a temperature of between 1100 and 1300° C.;

rolling the semifinished product such that a hot-rolled sheet is obtained, and that an end-of-rolling temperature at an end of the rolling is 890° C. or higher;

causing a delay between the end of the rolling and a start of a cooling such that a point defined by the delay and the end-of-rolling temperature lies within an area defined by the ABCD′E′F′A plot of FIG. 1 ; and

coiling the hot-rolled sheet at a temperature below 580° C., such that a coiled hot-rolled sheet is obtained, such that the coiled hot rolled sheet has a recrystallized fraction of greater than 75%, a tensile strength of greater than 900 MPa and a mean grain size of less than 18 microns; and

subjecting the coiled hot-rolled sheet to a cold deformation operation with an equivalent ratio of 30% or less,

wherein the steel from which the semifinished product is smelted comprises, in weight %, 0.5%≦C≦0.7%, 17%≦Mn<24%, Si≦3%, Al≦0.050%, S≦0.030%, P≦0.080% and N≦0. 1%, and further comprises iron and inevitable impurities resulting from the smelting.

9. The process as claimed in claim 8 , wherein the casting comprises casting a semifinished product between steel rolls into a thin strip.

10. The process as claimed in claim 8 , wherein the delay is caused such that the point defined by said delay and said end-of-rolling temperature lies within an area defined by the ABCDEFA plot of FIG. 1 , and that the hot-rolled sheet after the coiling and before the start of the cooling has the recrystallized fraction of 100%.

11. A process for manufacturing a cold-rolled steel sheet, comprising:

subjecting a steel sheet obtained by the process as claimed in claim 8 to at least one operation,

wherein each of the at least one operation comprises: cold-rolling the sheet; annealing the sheet at a temperature of between 600 and 900° C. for a period of between 10 and 500 seconds; and cooling the sheet after the annealing, at a cooling rate greater than 0.5° C/s such that a surface fraction of precipitated carbides is 0%,

the steel sheet before a first of the at least one operation has a mean grain size of less than 18 microns and a recrystallized fraction of greater than 75%, and

the steel sheet after being subjected to the at least one operation has a mean grain size in a range from 0.5 to 15 microns.

12. The process as claimed in claim 11 , further comprising:

performing a cold deformation operation after the at least one operation, such that an equivalent deformation ratio is of 30% or less.